Buildings provided with flat roofs are conventionally covered with a roofing assembly. This roofing assembly is supported by the generally horizontal planking of the roof. The top face of the roofing assembly is itself not horizontal, since it is desirable and in fact required to provide a slope to this top face, slope being directed toward a water drainage outlet port located usually centrally of the roof. Hence, as rainwater falls over the roof, it will slide toward and into the water outlet to escape from the roof, thereby preventing water accumulation thereon.
In conventional roofing assemblies, the slope of the top face thereof is made possible by the tapering of the rigid, flat, insulating batts, which form part of the roofing assembly, toward said water outlet port. This tapering of the insulating material of the roof is undesirable, since it means that heat retention capability of the roof is not of uniform distribution. This in turn means that heating costs for the building will be more difficult to control.